The invention relates to a method and a device for activating a physical or chemical reaction, enabling the kinetics of the reaction to be increased. It relates in particular to a process enabling precipitation of metals diluted in solutions, which consists in:
disposing the mixture in a reactor having two walls facing one another and close to one another, the mixture filling the space between the two walls and forming therein a layer of small thickness and of great length in a direction defined by a geometric axis parallel to the walls,
agitating the solution by activation of an agitating means arranged outside the reactor to act through said walls on an agitation zone covering a part of said layer and having a small dimension in the direction of the geometric axis,
and moving the agitating means so that the agitation zone spans appreciably the whole of said space located between the two walls.
The document EP-A-0,014,109 describes a process and a device enabling physical and/or chemical reactions to be fostered in a fluid medium by subjecting a magnetic substance, dispersed in the fluid and playing a physical and/or chemical role in the reaction to be fostered, to a variable magnetic field. The magnetic field is created by means of different electromagnetic coils arranged outside a recipient or reactor containing the fluid medium and the magnetic substance. The reactor is a revolution cylinder. The coils are preferably arranged on several levels of the reactor in the heightwise direction so that the application zone of the magnetic field covers a large portion of the reactor, with several electromagnets per level.
For certain of the physical and/or chemical reactions to be fostered by the process and device described above, the maximum linear velocity of the fluid in the reactor proves determinant for the efficiency of the reaction and has to remain fairly low. This is the case in particular for cementation reactions. What is referred to here as cementation is the process consisting in replacing a relatively noble metal MN present in a solution in ionic form by a more reactive metal MR introduced in solid form, according to a precipitation reaction of the type: 
In a reaction of this type, the kinetics of the process are a function of the surface offered by the solid reactive metal and of the noble metal concentration of the solution. It is therefore preferable to ensure rapid renewal of the solution in contact with the reactive metal so that the solution in the vicinity of the reactive metal is not depleted in noble metal ions. It is at the same time preferable to increase the reaction surface. However, if the size of the reactive metal particles is decreased too much in order to increase their reaction surface, it becomes difficult to ensure a sufficient relative flow velocity of the solution with respect to the reactive metal particles to prevent depletion of the solution mentioned above. Moreover, too high a flow velocity does not enable the solution to be treated in a single run, which means that the solution has to be passed several times over the same reactive metal bed, mixing it each time with non-treated solution. To achieve optimal kinematics and global efficiency, a compromise therefore has to be found between the size of the reactive metal particles and the relative velocity of the solution with respect to these particles. It is also necessary to prevent precipitation of the noble metal taking place at the surface of the reactive metal, for in this case the reaction would be quickly passivated.
When the activation process by electromagnetic fields described above is implemented within the scope of cementation reactions aiming to extract a noble metal such as copper, by means of iron used as reactive metal, application of an alternating magnetic field enables agitation of the solution and speeding-up of its kinetics to be achieved. However, the mean linear velocity of the solution in the active part of the reactor subjected to the magnetic field must, for the reasons explained above, remain within a range whose upper bound is low. To give a precise idea, if three levels of four pairs of electromagnets are used, as described in the document EP-A-0,014,109, with a solution containing 3 g/l of copper, the mean linear velocity of the solution is about 12 cm/s only.
Given this constraint, it is the cross-section of the active part of the reactor which determines the reactor flow rate. In a device of this kind however, the reactor cross-section is greatly limited by the power of the available electromagnets. In practice, the diameter used does not exceed 16 cm, whence a maximum flow rate not exceeding 10 m3/hr. These performances are far from those expected industrially for metallurgical processes if we consider that for an industrial installation enabling for example 5,000 tons of copper to be produced per annum from a solution containing 3 g/l of copper, a flow rate of 190 m3/hr is necessary, requiring with the technology described 20 reactors totalling 240 pairs of electromagnets. The high costs arising from the electromagnets should be underlined, which disqualify this type of technology. The electromagnets do in fact constitute an expensive item in the investment budget. Furthermore they have a high operating cost as they give rise to large energy expenses, not forgetting servicing and maintenance costs.
The exchange surface between the reactive metal and the solution has moreover been attempted to be improved by means of fluidised beds. An example of implementation of these fluidised beds is described in the Patent U.S. Pat. No. 3,154,411. In this embodiment, nearly 99% of the copper dissolved in a solution is extracted. However, the iron used reacts greatly with the acidity of the medium with the consequence of a large amount of hydrogen being given off and a reduced iron yield. Moreover, this process is not continuous and the copper cements are rich in iron. Furthermore, Swiss Patent No. 9827/72 discloses that the difficulties proper to fluidised beds can be overcome by performing cementation of metals such as Cu, Cd, Co, etc. on zinc granules fluidised in a mechanically agitated reactor. In this embodiment, the exchanges are excellent and the precipitated metals are driven out of the fluidised bed whereas the larger zinc granules stagnate there until they reach a very small size. The drawback of this system lies in the difficulty of implementing reliable mechanical agitation in a tubular reactor of large height. Any mechanical system placed in such conditions is chemically attacked and abraded by the cements. To operate, these systems have to implement delicate embodiments such as bearings kept constantly under pressure of a pure and neutral solution.
The document U.S. Pat. No. 5,227,138 relates to a device designed to displace a biological liquid in a capillary tube wherein a ferromagnetic piston driven externally by a permanent magnet is made to move. This device is intended for biological uses.
The document U.S. Pat. No. 5,222,808 describes a mixture of two liquids in a capillary tube. It makes use of a magnetic agitation system using one or more magnetic cores moved by a variable external magnetic field. The magnetic cores are formed by microscopic powders which are directed in the field lines forming aggregates. This device is also intended for biological uses.
The object of the present invention is to reduce the drawbacks proper to the remote activation technologies described above. Its object is to achieve a cementation process of metals with optimum yield. Its object is to propose an installation with a high unitary processing capacity. Its object is also to enable greater agitation of the fluid solution involved in the reaction to be activated while limiting the number and cost of the activation means.
The activation process according to the invention is characterized in that:
solid ferromagnetic particles fluidised in the current of the solution are used, being the seat of a deposition when the cementation reaction takes place, the particles having a predetermined granulometry,
the solution is injected via the bottom of the reactor causing an ascending flow of the solution in a vertical direction parallel to the walls, whereas the solid ferromagnetic particles are introduced at the top part of the fluidised bed.
A The impacts caused between the solid ferromagnetic particles and the reactor walls enable the metallic deposits to be detached continuously.
Preferably, the agitating means is moved in an alternating movement between a first extreme position and a second extreme position situated in such a way that the agitation zone is able to appreciably span the whole of said portion of space. The movement of the agitating means can be limited to a to-and-fro translation movement. The processing capacity is high, as there is no limitation of the cross-section of the reactor, which can have the required width or diameter to treat a given flow rate of solutions while preserving a limited air-gap. The agitating means comprise a plurality of electromagnets sequentially supplied with periodic currents to create an electromagnetic field able to direct the ferromagnetic particles alternately in two distinct directions. The thin layer of fluid in the active zone enables a maximum effect of the magnetic forces to be obtained while also having a high flow rate which is not possible in cylindrical embodiments of the state of the art.
Alternatively or cumulatively, other agitating means can be provided, for example at least one ultrasonic transducer, at least one of said walls being lined with a flexible membrane containing a gel able to transmit the ultrasounds, said transducer having a head in contact with said flexible membrane.
According to another feature of the invention, the object of the latter is also to achieve a device for implementation of the process described above and comprising means for introducing the solution via the bottom of the reactor causing an ascending flow of the solution in a vertical direction parallel to the walls, whereas the solid ferromagnetic particles are introduced at the top part of the fluidised bed.
According to an alternative embodiment, the device comprises means for injecting reactive metal wires able to be used for injecting liquid chemical adjuvants.
Preferably, both of the walls are shaped in such a way that their external surface is geometrically defined by a set of segments of straight lines parallel to one and the same geometric axis and bearing on any curve extending in a plane perpendicular to said axis, the distance between each segment of one of the walls and the other wall being constant. This geometrical definition covers in particular the case where both the walls are flat or cylindrical with a circular base.
Preferably, the device comprises in addition drive means for driving the fluid in a driving direction parallel to said geometric axis, the walls comprising, on their faces facing one another, asperities forming restrictions designed to cause local accelerations of the fluid.
The invention is mainly applicable to cementation of non-ferrous metals, both in the primary metallurgy sector and in that of decontamination of ground surfaces and of solutions charged with heavy metals.